base-tests: use subshells to manage current directory
base-tests: use subshells to manage current directory

It saves on having to pair `cd <path> && think && cd ..`.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust